SuperLez:
Doubt that a 10 or 12 tonner will enable you to pass a Class C test nowadays - vehicles now require 8 forward gears and I doubt that boxes like these would be found anything less than a 16 tonner.
You can still do your cat C test in a 12 tonner, thats exactly what I did 3 year ago. The wagon was a 54 reg eurocargo box wagon with a slapover 8 box.
When I did my C+E, they stuck an 8ton drawbar box on the back giving a train weight of 20t, and it was unladen so probably only about 12t altogether
This allowed me to then drive a fully freighted 44tonner 